Processing order

The order of processing is often significant when evaluating DITA content. Although the DITA specification
does not mandate a specific order for processing, the DITA-OT has determined that performing filtering before conref
resolution best meets user expectations. Switching the order of processing, while legal, may give different results.

The DITA-OT project has found that filtering first provides several benefits. Consider the following sample
that contains a <note> element that both uses conref and contains a @product
attribute:

<note conref="documentA.dita#doc/note" product="MyProd"/>

If the @conref attribute is evaluated first, then documentA must be parsed in order to retrieve the note
content. That content is then stored in the current document (or in a representation of that document in
memory). However, if all content with product="MyProd" is filtered out, then that work is all discarded later in
the build.

If the filtering is done first (as in the DITA-OT), this element is discarded immediately, and
documentA is never examined. This provides several important benefits:

Time is saved by discarding unused content as early as possible; all future steps can load the document
without this extra content.

Additional time is saved case by not evaluating the @conref attribute; in fact, documentA does not even
need to be parsed.

Any user reproducing this build does not need documentA. If the content is sent to a translation team,
that team can reproduce an error-free build without documentA; this means documentA can be kept back from
translation, preventing accidental translation and increased costs.

If the order of these two steps is reversed, so that conref is evaluated first, it is possible that results
will differ. For example, in the code sample above, the @product attribute will override the product setting on
the referencing note. Assume that the <note> elements in documentA is defined as follows:

<note id="note" product="SomeOtherProduct">This is an important note!</note>

A process that filters out product="SomeOtherProduct" will remove the target of the original conref before that
conref is ever evaluated, which will result in a broken reference. Evaluating conref first would resolve the
reference, and only later filter out the target of the conref. While some use cases can be found where this is
the desired behavior, benefits such as those described above resulted in the current processing order used by
the DITA-OT.